The present invention relates to a cruise control apparatus for a vehicle, and more particularly to such an apparatus which is capable of preventing cruise control from being set when certain conditions are met.
There has been known a conventional cruise control apparatus for a vehicle of the type in which the target speed at which the vehicle is to travel is determined as the speed of the vehicle sensed by a speed sensor at the instant when the operator turns on a cruise control switch. The target speed thus determined is then stored in a RAM in a microcomputer. Based on the present speed as continuously sensed by the speed sensor every second and the target speed, an amount of opening of a throttle valve in the intake manifold of the engine which corresponds to the target speed is calculated by the microcomputer. Then, the throttle valve is set to the thus calculated opening amount through the action of an actuator so that the speed of the vehicle is adjusted to the target speed. Thereafter, when the operator steps on a brake pedal for brake application, a stop lamp switch, which is operated upon brake application, is turned on to release or stop cruise control, and at the same time a brake switch is turned off to interrupt the supply of power from a source of electricity to the actuator. Subsequently, when the brake pedal is released, the stop lamp switch and the brake switch are operated to return to the initial condition so that the vehicle is ready for travelling under cruise control.
With the conventional cruise control apparatus constructed in the above manner, if either one of the stop lamp switch and the brake switch has failed, it is still possible to cancel cruise control. However, there is the problem that if both switches fail after the cruise control setting switch has been operated to set cruise control, it becomes impossible to cancel cruise control. The failure of either one of the stop lamp switch and the brake switch can be detected through the resulting disagreement in operational mode between these switches so that the setting of cruise control can be inhibited upon detection of such a failure. In this case, however, if there is disagreement between the operating points of these switches, as may occur when the operator lightly steps on the brake pedal, a failure in one of the switches will be detected, despite the fact that both of the switches operate without any problem, and thus the setting of cruise control will be incorrectly inhibited.